April 12, 2007

Photosynthesis

Green Quanta

Quantum effects could account for fast energy transfer

Celia Henry Arnaud

A quantum mechanical effect could explain why energy transfer is so efficient in photosynthesis, according to a new study.??

During photosynthesis, light-harvesting proteins absorb and transfer solar energy via a series of precisely placed chromophores. In the usual picture of energy transfer, the system hops from one excited state to the next until it reaches the final state. Now, scientists have found the first direct evidence that the excitation moves with a coherent wavelike motion rather than a hopping motion.

Chemistry professor Graham R. Fleming, postdoc Gregory S. Engel, and coworkers at the University of California, Berkeley, and Lawrence Berkeley National Laboratory use two-dimensional electronic spectroscopy of a bacterial photosynthetic complex to show that a quantum mechanical effect called quantum beating is at play in photosynthesis (Nature 2007, 446, 782). In quantum beating, the system appears to move back and forth between coupled electronic states. "We're presenting the first direct evidence for seeing this effect in a photosynthetic system," Engel says.

Such electronic coupling lets the system sample many energy-transfer pathways simultaneously. The way the photosynthetic system determines which pathway to take could be analogous to quantum computing, Engel says. "The photosynthetic complex has all the components that it would need to behave as a quantum computational device," he says. "This may help to explain the incredible efficiency."

The researchers measured the bacterial complex's electron spectra only at low temperatures. But they expect that the quantum effects are also important under physiological conditions because the system's underlying chemistry and physics don't change with temperature, Engel says.

"We're getting at a basic design principle that's in play in these photosynthetic systems," Engel says. "It's a mechanism that we hope may lead to synthetic systems that use similar principles in the future." Development of artificial light-harvesting devices has been stymied by poor efficiency.

Only a few people had suspected that quantum effects might be a key driver of photosynthetic efficiency, according to Rienk van Grondelle, a biophysicist at Free University, in Amsterdam. "Ninety-nine percent of the photosynthesis energy transfer universe would never have believed this to be relevant in the absence of these experiments," van Grondelle says.

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